idlerboat 116 Posted January 25, 2011 Share Posted January 25, 2011 ....and then some of us have steel boats...... Link to post Share on other sites
Grinna 2 Posted January 25, 2011 Share Posted January 25, 2011 ....and then some of us have steel boats...... Or concrete with steel mesh running through it. I bet that has the potential to play havoc with a compass. Actually, how do they get around the issues of steel in the hull and superstructure affecting the compass?? Must be possible. Link to post Share on other sites
PaulR 3 Posted January 25, 2011 Share Posted January 25, 2011 One ancient problem was the little transistor radio in the cockpit would alter the compass a lot. Don't know about these new fangled mobile phones, ipods, apples or blackberries etc. A fire extinguisher on the other side of the bulkhead has a massive effect, especially after upgrading from a 1.okg to a 2.5kg as per new Safety Regs. Link to post Share on other sites
Guest Posted January 26, 2011 Share Posted January 26, 2011 fellas go and get your compasses swung...steel, ferro, fire extinguishers on bulkheads, steel chastity belts mounted on the bulkhead etc can all be adjusted for, there is no reason to have a devaition of more than 2-3 degrees and then only at a couple of points on the dial...then keep the trannies, mobile, ipods etc away from the compass and you will have a beautiful life!! Link to post Share on other sites
Guest Posted January 27, 2011 Share Posted January 27, 2011 No need for me, I can't sail in that straight a line :) Link to post Share on other sites
idlerboat 116 Posted January 27, 2011 Share Posted January 27, 2011 Actually, how do they get around the issues of steel in the hull and superstructure affecting the compass?? Must be possible. With a steel boat compass These expensive bits of gear have balancing or compensating "balls" as well as a series of smaller magnets. Depending on the boats field, magnets are added or subtracted in certain positions under the compass. Steel boats that sit in one spot for an extended period of time can have its magnetic field become almost stable, and therefore the compass can be adjusted to a reasonably high degree of accuracy. The problem is when the boat moves.........As it changes location, its field can rearange itself, needing a re adjustment of the compass ...which is where compass adjusting piles in major harbours used to be so important. These days with electronic fluxgate compasses, just going slowly around in a couple of circles allows the electronics to sense the anomilies and correct them.... Magic Link to post Share on other sites
SloopJohnB 322 Posted January 27, 2011 Share Posted January 27, 2011 Steel boats that sit in one spot for an extended period of time can have its magnetic field become almost stable, and therefore the compass can be adjusted to a reasonably high degree of accuracy. The problem is when the boat moves.........As it changes location, its field can rearange itself, needing a re adjustment of the compass ...which is where compass adjusting piles in major harbours used to be so important. These days with electronic fluxgate compasses, just going slowly around in a couple of circles allows the electronics to sense the anomilies and correct them.... Magic Also the direction the keel is laid affects the magnetic fields built into an steel hull. Link to post Share on other sites
grant 40 Posted January 27, 2011 Share Posted January 27, 2011 These expensive bits of gear have balancing or compensating "balls" as well as a series of smaller magnets. Lord Kelvin's Balls, if memory serves correctly he must have walked with a wide stance..... Link to post Share on other sites
Guest Posted January 27, 2011 Share Posted January 27, 2011 Bugger the deviation card, there's bound to be a iPhone App that will do it for you. Big Steel I used at Xmas uses the 4 rod magnet system, works very well on the total top to bottom steely. Having issues with the autodriver compass though and that's a fluxgate, the bastard thing. Link to post Share on other sites
Guest Posted January 28, 2011 Share Posted January 28, 2011 i remeber being asked after a akl-tuaranga race for the dev card fopr rnb, the guy asked and the entire crew responded, "+2 degress all the way around....." he looked on in disblief.. but was ok about it.... Link to post Share on other sites
rigger 47 Posted January 28, 2011 Share Posted January 28, 2011 Below is an excel work sheet, where the text is blue enter the observed bearing of one object taken on the different headings. You do not need to know the true bearing of the object for this one so can do without the need of a chart. Had to create it for a course several years ago. I did pass. Hopefully it is the error free file Link to post Share on other sites
Guest Posted January 28, 2011 Share Posted January 28, 2011 More explanation of the technique please Rigger, that looks seriously cool. Link to post Share on other sites
rigger 47 Posted January 28, 2011 Share Posted January 28, 2011 More explanation of the technique please Rigger, that looks seriously cool. I'll try and dig out my notes tomorrow, has been a long long day..... Link to post Share on other sites
idlerboat 116 Posted January 28, 2011 Share Posted January 28, 2011 ....rigger you de man ! so sorry i didnt have a chance to meet up with you last visit.. Next time Link to post Share on other sites
Fusion 0 Posted January 29, 2011 Share Posted January 29, 2011 Bugger the deviation card, there's bound to be a iPhone App that will do it for you. Big Steel I used at Xmas uses the 4 rod magnet system, works very well on the total top to bottom steely. Having issues with the autodriver compass though and that's a fluxgate, the bastard thing. KM I have been playing with my Fluxgate compass on fusion and have the best results 2m up the Mizzen mast and the gyro directly in line below on the bulk head a water level. The performance has been greatly improved in a following sea. Link to post Share on other sites
rigger 47 Posted January 30, 2011 Share Posted January 30, 2011 Ok the excel file I posted earlier uses the bearings taken and calculate 5 coeffiecints that it then uses to work out a deviation curve Coef A - is the sum of the deviations divided by the number of observations - 8 Coef B - is the E deviation - the W deviation then divided by 2 Coef C - is the N deviation - the W deviation then divided by 2 Coef D - is the NE deviation - SE deviation + SW deviation - NW deviation then divided by 4 Coef E - is the N deviation - E deviation + S deviation - W deviation then divided by 4 the using the formula Deviation for heading = Coef A + (Coef B*SIN(Heading))+(Coef C*COS(Heading))+(Coef D*SIN(2*Heading))+(Coef E*COS(2*Heading)) Much easier to get a computer to do it. Then again you can just observe the deviation on each heading and sketch a curve and you would be good. Correcting the compass is another thing. Link to post Share on other sites
NevP 0 Posted January 30, 2011 Share Posted January 30, 2011 rigger, I'm a little slow on the uptake here. Can you describe how the readings are taken in more detail? Am I correct that object being sighted needs to be far enough distant that as the boat moves on each heading the bearing change is insignificant? I assume the compass bearing to be recorded needs to be taken from the ship's compass that is being calibrated and not from a hand held compass. How do you observe a bearing to an object from the ship's compass? Link to post Share on other sites
rigger 47 Posted January 31, 2011 Share Posted January 31, 2011 rigger, I'm a little slow on the uptake here. Can you describe how the readings are taken in more detail? Am I correct that object being sighted needs to be far enough distant that as the boat moves on each heading the bearing change is insignificant? I assume the compass bearing to be recorded needs to be taken from the ship's compass that is being calibrated and not from a hand held compass. How do you observe a bearing to an object from the ship's compass? The bearings are taken of a distant object / point and taken from the compass you want to make a deviation card for - so yes te ship's compass. If you really wanted to you could make a deviation card for a h/h compass as long as the handheld compass is going to be held in the one location everytme it is used, on the card you would describe where and at what height the compass needs to be held.... but I did try it and had a play with a couple of h/h compasses and cards and found it was a waste of time, too many other errors / difficulties. Normally the ship's compass is sited in a location that gives 360degree line of site, well as near as you can - normally you have a few minor blind spots. Most yachts will have obstructions. To get around blind spots / obstructions you can have mountings for a pelorus and use that to get around the blind spots / obstructions. For the work sheet I posted you need 8 bearings of the distant object taken from the same (or as near as possible) location, N, NE, E, SE, S, SW, W, NW, then by using the work sheet or calculation you can determine the deviation for a range of headings, 10 degree increments are all that is needed for a ship and should more than cover you on a yacht. I'm no expert on correcting compasses, it was part of my training and we had to be able to correct a compass to have 4deg or less maximum deviation - was an enjoyable course and unfortunately I have forgotten a lot of the theory, need to dig my hand written notes out.... Link to post Share on other sites
Guest Posted January 31, 2011 Share Posted January 31, 2011 Nev, you point the vessel at the object and read the compass bearing. I hope I've not wrongly assumed you've missed this point. Link to post Share on other sites
rigger 47 Posted January 31, 2011 Share Posted January 31, 2011 Nev, you point the vessel at the object and read the compass bearing. I hope I've not wrongly assumed you've missed this point. If you are using a number of distance points you can do that but if you are using only one which is what you do with the method I was describing in the work sheet and with the equations that is not possible - only one of the 8 bearings required may allow you to point the vessel at the object, the other 7 the vessel will be on a heading different to the bearing towards the object. To assist taking the bearings you can use: a shadow pin, a bit of string, an azimuth ring - unfortunately a very large number of small boat compasses are only any good for steering not taking bearings as the are bulkhead mounted and have no easy way to take a bearing without pointing the vessel at the object. Link to post Share on other sites
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